1. Field of the Invention
The present invention relates generally to nanomechanical memory devices, and relates more particularly to a nanomechanical device that exhibits discrete states under specific stimuli.
2. Description of Related Art
For many years, semiconducting devices have been used to fabricate memory arrays or storage units. The fabrication of such devices on a micrometer scale has provided a number of attractive advantages in reducing the size and power of large scale integrated circuits. Indeed, with each new semiconducting device generation, smaller, reduced power and typically faster devices are produced.
However, such advances will not go on indefinitely. The National Technology Roadmap for Semiconductors postulates that due to physical and economic reasons, the current scaling advances will continue only until the year 2010 or so. Transistor elements used in processors face serious obstacles, including excess heating, power requirements, and tunneling effects.
Memory can be broken up into two general groups: volatile and non-volatile. Volatile memory does not retain state information once power to the memory element is turned off. However, volatile memory has several advantages and is typically used for fast access or swapping of information between the processing and storage elements. Non-volatile memory, on the other hand, retains state information until it is changed, and is typically used for longer term storage.
Volatile memory is often provided as DRAM (Dynamic Random Access Memory) chips on computer motherboards for fast access. Non-volatile memory is often seen in the form of flash memory and hard drives, each of which may be slower than DRAM, but provide more robust long term storage. Flash memory is similar in structure to RAM and is typically composed of electro-capacitive elements. Hard drives are often formed with paramagnetic islands used to store information.
Regardless of the form a memory cell takes, present realizations of computer memory face two serious challenges involving scaling and fragility. As electronic memory elements are made smaller, significant challenges arise with respect to processing individual components. With regard to magnetic memory elements, packing density is limited by the superparamagnetic limit. The superparamagnetic limit is the point at which individual memory elements or bits begin to interact with each other and lose independence with respect to individual state information.
The issue of fragility refers to the susceptibility of electro-capacitive memory and processor elements to electromagnetic radiation and particle discharges. Magnetic elements also can be altered in the presence of large magnetic fields. In addition, magnetic hard drives involve platters and read/write heads which are susceptible to impact shock.
One type of memory element that addresses the issues of scaling and fragility is a micromechanical memory element. Micromechanical memory elements have greater packing density, or a smaller size, while decreasing fragility of the device. Micromechanical memory elements are known and have been profiled in numerous other documents, notably U.S. Pat. No. 4,979,149 (Popovic et al.) and U.S. Pat. No. 5,774,414 (Melzner et al.), and references therein. However, these types of memory elements with critical dimensions in the micron range have not been able to achieve either the packing densities or read/write speeds that would make them competitive with conventional electro-capacitive or magnetic devices in present commercial settings.
One advantageous feature of a memory element, mechanical or otherwise, is the existence of multi-state stability in the device. Typical devices exhibit bistability, where the device is in one state to define a “1” and another state to define a “0”. The two states may then be used to perform binary computation in a computing engine. A critical function of these devices is the ability to read or change the state of the device. In the case of electro-capacitive elements, such as the conventional RAM and ROM memories, reading and writing states involves the addition or subtraction of electronic charge. In mechanical elements, state reading and writing is typically resolved through the manipulation of the element into two distinct positional states. Micromechanical elements have been fabricated with enough intrinsic compressive stress to place them into a condition of bistability manifested by a convex or concave buckling effect. This is a static bistable condition and cannot be easily changed once the device is fabricated.
The read/write procedure for the bistable mechanical memory elements described above is straightforward and intuitive, perhaps because of the mechanical features of the element. In the case of mechanical elements that have critical dimensions in the sub-micron range, however, positionally distinct states are a non-optimal solution. The reduced usefulness of positionally distinct states for elements having sub-micron dimensions detracts from the ability of the elements to achieve competitive read/write speeds and packing densities. In contrast, nanomechanical elements are small enough to achieve competitive packing densities, and their small size leads to an intrinsically high natural frequency of motion, allowing for very fast read/write times. See, e.g., U.S. Pat. Nos. 6,495,905, 6,574,130, 6,781,166 and 6,548,841.
Nanomechanical elements tend to possess features that preclude easy monitoring of their positional states. Their surface-to-volume ratio is higher than that of micron-size or millimeter-size devices of the same geometry, which leads to a greater sensitivity to both friction and sticking effects. This greater sensitivity prevents physical manipulation of such nanomechanical devices, as such forces are often strong enough to cause failure or destruction during manipulation. Additionally, due to their intrinsically high stiffness, nanomechanical elements often possess very small amplitudes of motion, even when subjected to very large forces. Stiffness typically scales inversely with system size and therefore proportionally to the natural frequency of the device. For example, a device with a natural frequency in the GHz range can often possess such high stiffness that it exhibits an amplitude of motion that may only be in the range of picometers (pm or 10−12 m).
The desired specifications for speed and size of nanomechanical elements compete with measurement of the positional state of the element. It is thus desirable to operate nanomechanical memory elements in a manner that is fundamentally different from micromechanical elements.
It would be desirable to obtain a nanomechanical device without positional or static bistability. It would further be desirable to obtain a nanomechanical memory device that is not fabricated with the addition of compressive forces to form an intrinsically bistable device.